For example, in order to measure color tone and brightness of a measured object surface such as a steel plate surface, it is general that an image of the surface is captured by utilizing a color camera, to check color hue, saturation, and brightness of the obtained image (RGB, CIE L*a*b*, etc.). However, for example, when a measured object is a steel plate whose color tone and brightness slightly change as the illumination and observation angles change, such as titanium and a part of stainless steel plates, it is unable to observe their change by using a certain constant illumination angle and observation angle. Also, when the measured object is a metal surface, the metal surface has a large contrast between specular reflection and diffuse reflection, and thus a standard RGB camera generates an error in color information, and it is difficult to observe subtle color shade.
From the above reason, a spectral reflectance is measured while changing the wavelength of the illumination light that forms an image in the camera as in Patent Literature 1 below, and the spectral reflectance is measured by utilizing a spectrophotometric colorimeter while changing the angle of the illumination and the camera, in order to accurately measure the color tone and the brightness of the measured object surface.